1. Field of the Invention
The present invention relates to a steam blower box for applying steam onto a material web, particularly paper, which travels past the steam blower box. The steam blower box has a front and a rear limitation as seen in the travel direction of the material web and a steam chamber located between the front and rear limitations, wherein the steam chamber is open toward the material web. The steam chamber is closed off at its front and rear ends in the direction of travel by a limiting surface each which protrudes in the direction toward the material web, and particularly forms a limiting edge; between the limiting surfaces, the steam chamber has a housing wall which is recessed relative to the material web. The invention further relates to a corresponding method for controlling the steam quantity and/or steam outlet speed from the steam outlet openings of a steam blower box.
2. Description of the Related Art
In certain process steps during the manufacture and further processing of paper, steam is supplied to a moving material or paper web. For this purpose, steam is usually blown in the towards the material web from a steam blower box which is arranged in the vicinity of the web but does not contact the web. For blowing the steam, the steam blower box has, on a side facing the web, steam outlet openings to which controllable quantities of steam are supplied in a suitable manner. The steam emerging from the steam blower box is supposed to impinge upon the material web, to condense on the material web and transfer the condensation heat to the material web. The condensate is simultaneously deposited on or in the material web and thereby produces an increase in moisture. Because of the movement of the material web, the condensate is continuously transported away from the steam application zone, so that no condensate layers can be formed which could significantly impair the further condensation because of its thickness.
However, because of friction at its surface, the moving material web produces an air flow which in the area near the web is taken along and conveyed into the steam application zone or steam chamber. The air in the steam application zone between the steam outlet openings of the steam blower box and the material web leads to difficulties in several respects.
First, the air present in the steam application zone comes into contact with the emerging steam, so that in the border area with the steam, the air is heated to 100° C. and is saturated with moisture. When cooling takes place later the air is oversaturated with moisture and discharges water droplets. This results in visible steam clouds which reduce the quality of the ambient area in the work area and lead to the formation of drops at the machine components. In addition, the energy utilized for heating the air is essentially lost for heating the web and the efficiency of the plant is reduced.
A second, significantly more important disadvantage of the presence of air in the steam application zone is that the heat transfer to the material web is reduced. While air molecules heated to 100° C. are cooling themselves when they transfer heat to the colder material web, so that the heat transfer drops immediately, the steam molecules transfer the entire condensation energy at the temperature level of 100° to the material web. This is the explanation for the known fact that the heat transfer as a result of condensation is significantly poorer when inert gases are present than in a steam application zone which is entirely filled with steam.
For displacing the air from the web surface and to produce a direct contact of the steam with the material web, so called high-speed steam blower boxes have been known in the art for some time, wherein steam jets which impinge with a sufficiently high speed on the material web destroy the air layer in the area near the surface of the material web and thereby produce a direct contact between steam and web molecules. However, this does not at all displace the air from the steam application area. The occurring negative pressure in the area of the steam outlet openings of the steam blower box is due to an injection effect, any air which has reached the steam application area is sucked up by the steam jets and is blown together with the steam toward the material web. Accordingly, in these known high-speed steam blower boxes, inert gas is also present in the steam application zone.
A device for applying steam onto a material web of the above-described type is known, for example, from DE 37 01 406 A1. In this device, a steam application chamber is to be sealed off by means of steam locks. The steam locks are produced by blower openings which are provided in the entry zone and the exit zone, wherein the blower openings are inclined toward the principal zone of the blower openings and through which the steam jets blow out. This steam lock also does not make it possible to reliably prevent the entry of air into the steam application chamber; this is because air is taken in because of the injector effect at the outlet openings of the steam jets. The presence of air is even described as being an advantage. It is also not sufficient, as partially known in the art, to align the steam jets obliquely against the travel direction of the web. As long as air is sucked up at the steam outlet openings, this air is conveyed into the up at the steam outlet openings, this air is conveyed into the steam application are and decreases the efficiency of the steam application.
In addition, a device for sealing a steam blower box is known from DE 297 03 627 U1. In this device, the blower chamber is to be sealed off by a double slot nozzle, wherein steam flows out of the slot-shaped nozzle facing the lower chamber and air flows out of the nozzle facing away from the lower chamber. When they impinge upon the material web, these two flows are supposed to separate in such a way that the air flow is conducted against the travel direction to a discharge means and the steam reaches the lower chamber. Aside from the fact that this device requires a high-pressure steam nozzle and a high-pressure air nozzle, a separation of steam and air cannot be achieved when additional air is discharged. The more air is blowing out of the high-pressure air nozzle, the higher the backed-up pressure of the jet impinging upon the material web will be. However, the relation between the pressure in the air area and the pressure in the steam are in the vicinity of the web is the deciding factor whether air enters the steam are or steam is discharged. This makes it clear that an additional air flow is counterproductive for keeping air out of the steam application zone or the steam chamber.